Conventionally, various types of electric storage devices have been known. For example, an electric storage device including an electrode assembly including a pair of mutually opposed electrodes, and an electrolytic solution impregnated into the electrode assembly has been known. In this electric storage device, the positive electrode of the electrode pair includes a positive-electrode active material layer containing a positive-electrode active material, and the negative electrode of the electrode pair includes a negative-electrode active material layer containing a negative-electrode active material.
As this type of electric storage device, there is known, for example, an electric storage device in which an electrolytic solution contains lithium hexafluorophosphate (LiPF6) serving as electrolytic salt, and a positive-electrode active material layer contains lithium carbonate and lithium metal oxide serving as a positive-electrode active material (JP 2001-167767 A).
In such an electric storage device, an acidic substance which causes metal contained in a positive-electrode active material to liquate out into the electrolytic solution is produced from LiPF6. The acidic substance produced from LiPF6 can be neutralized, however, by the basic lithium carbonate. Accordingly, the metal contained in the positive-electrode active material can be suppressed or prevented from being liquated out into the electrolytic solution by the acidic substance. Consequently, it is possible to suppress or prevent battery capacity from degrading after high-temperature storage.
In such an electric storage device, however, a gas is generated due to the reaction between the acidic substance produced from LiPF6 and the lithium carbonate. Since the lithium carbonate is simply dispersed in the positive-electrode active material layer in the electric storage device, such a gas is also generated inside the electrode assembly and may accumulate therein. If the gas accumulates inside the electrode assembly, the electric storage device, after being stored at particularly high temperature, may increase in resistance or decrease in electric capacity.